I think this is pretty normal to be fair. So long as its not really loud your ok. When was the diff oil last changed? Getting the fluid drained and refilled with the BMW upgraded oil will also help with the noise if it really bothers you. Part number - 83 22 2 282 583, SAF-XJ plus FM Booster. Comes as a 3 bottle kit and is used to get rid of that low speed grinding noise you get on tight 1st gear corners. Not the cheapest oil though.